As an electrostatic latent image development for electrophotography, one-component magnetic jumping development, one-component non-magnetic contacting development, and two-component development, in which development is performed by frictionally electrifying a toner, transporting a developer, and contacting with an electrostatic latent image, by mixing an insulating non-magnetic toner and magnetic carrier particles, are known so far.
Particularly attention has been paid gain to application of the two-component development to the color printer as a promising method in near future.
A particulate carrier, which is used in such two-component development, is usually produced by coating a magnetic carrier core material with an appropriate material in order to prevent filming a toner onto the surface of the carrier, to form a carrier-uniform surface, to elongate the lifetime of a developer, to prevent damage of a sensitizer by a carrier, to control charge quantity, or for other purposes.
Conventional resin-coated carriers are not, however, satisfactory in durability because the coat is easily exfoliated by an impact such as stirring applied when used or for other reasons.
To solve this problem, the inventors developed and proposed a method to form a polyolefin-based resin coat by directly carrying out polymerization of an olefin-based monomer on carrier-core-material particles of materials such as ferrite, as described, for example, in Japanese Patent Laid-open Pub. No. Hei. 2-187771. The polyolefin-based resin-coated carrier obtained according to this method, 1) has the strong adhesion strength between the core material and the coat, 2) does not give any deterioration in the quality of image, 3) is excellent in durability, and 4) is excellent in spent stability, even if copying is repeated continuously for a long time, because the coat is directly formed on the carrier core material particles.
On the other hand, however, this polyolefin-based resin-coated carrier did not have adequate durability, not only because control of charge polarity and adjustment of charge quantity can not be freely conducted, but also because of the problem that the external additives are spent by attachment of external additives of a toner or for other reasons.
In addition, the carrier did not have enough properties which allow fine adjustment of resistance and adjustment of image density.
As methods to solve the above-mentioned problems, a method to improve charge quantity by containing nigrosin in a carrier-coated resin is disclosed in Japanese Patent Laid-open Pub. No. Sho. 53-100242, a method to improve fluidity by adding a fluidity-improving agent is disclosed in Japanese Patent Laid-open Pub. No. Sho. 61-9661, and a method, to prevent making the charging property uniform and being spent by adding one selected from a group consisting of electroconductive fine particles, inorganic filler particles, and a charge-controlling agent, is disclosed in Japanese Patent Laid-open Pub. No. Hei. 2-210365.
These methods, however, could not satisfy both 1) freely controlling charge polarity, adjusting charge quantity, and adjusting resistance, with taking advantage of an excellent property that the above-mentioned polyolefin-based resin-coated carrier has, and 2) preventing external additives of a toner from being spent.
While, the carriers produced by coating magnetic particles with a resin are disclosed in U.S. Pat. Nos. 5,079,124, 5,085,963 and 5,652,060. All of these carriers, however, use globular magnetic particles as the carrier core material provided with magnetism which is used in the present invention. These carriers not only nonuse convex polyhedron magnetic particles but also are provided with no outermost layer containing convex polyhedron magnetic particles. The object of the present invention in which the convex polyhedron magnetic particles are added to the outermost layer to secure easy control of charge quantity is unattainable by the technologies disclosed in these patent publications.
In U.S. Pat. No. 5,641,601, a toner provided with convex polyhedron magnetic particles stuck to the surface thereof is disclosed. However, this toner is not used for a carrier. Also, even if this toner is combined with the invention disclosed in U.S. Pat. No. 5,079,124, the structure of the invention cannot be attained. Hence, the object of the present invention to secure easy control of charge quantity is unattainable by the technologies disclosed in U.S. Pat. No. 5,641,601.
The present invention aims to solve the above-mentioned problems, i.e. the purpose of the present invention is to provide a carrier for electrophotography and a developer using the carrier, which allows adjusting the charge quantity and static resistance freely, with taking advantage of an excellent property that a carrier having a polyolefin-based resin coat has, obtaining an image stable in light and shade, and effectively preventing external additives from being spent by attachment of external additives of a toner.